1. Related Field
The present invention relates generally to an imaging system that can image the surface details of a workpiece, such as a rolled/drawn metal bar.
2. Description of the Related Art
It is known to produce a metal bar by a mechanical process such as rolling or drawing. Such metal bar is different than a metal slab, bloom, or strip (hereafter referenced as Metal Flat) in that the cross section of such a bar has a smaller circumference/cross-section-area ratio such that the bar may rotate/twist about a longitudinal axis while moving forward longitudinally. For example, the bar shapes shown in FIG. 2 have a ratio of circumference to cross-sectional-area that is equal to or smaller than 4.25 when the cross sectional area is unity for the given shape. The shape, when taken in cross section, of such a metal bar may be a round shape (item 102), an oval shape (item 104), or a polygonal shape, as shown as a hexagon (item 106), octagon (item 108) or a square (item 110) in FIG. 2. Furthermore, such a metal bar is substantial in length. The length to circumference ratio is typically over 10 and the length to cross-section critical dimension (such as the diameter of a round bar or the side width a square bar) is over 30. A metal bar of this type is typically referred to as “long products” rather than “flat products” in the related industries. Rolling, drawing, extrusion and the like, as used in this disclosure and hereafter referenced as a Reducing Process, describe the ways for reducing the cross sectional dimensions of the metal workpiece through mechanical contact of the applicable tools, such as rollers and drawing dies, and the workpiece. These Reducing Processes are generally continuous, or substantially continuous, in nature.
In the metal production industry, the presence or absence of surface defects is a relevant criterion upon which assessments of the metal products are made. For instance, surface defects account for half of the external rejects (i.e., rejected by the customer) for the steel bar and rod industry. However, the conventional art provides no reliable means to detect such defects. There are several challenges that conventional inspection approaches have been unable to overcome.
First, in the case where inspection occurs while the metal bar products are “hot,” the temperature can be as high as 1,100° C., preventing the use of many inspection technologies. Second, the traveling speed of such a metal bar along its longitudinal axis as described above can be, presently, as fast as 100 m/s, several times faster than the speed of the fastest metal strip and nearly 100 times faster than a metal slab or bloom. Further, speed increases are expected in the near future in the range of 150 m/s to 200 m/s. Conventional inspection approaches simply cannot accommodate such high traveling speeds. Third, a high temperature metal bar such as described above is typically confined in a sectional conduit so that the bar will not cobble. Cobbling is an incident wherein a hot, high speed metal bar runs freely outside the conduit. The space, therefore, for any inspection device is extremely limited. Last, the length of such a metal bar, together with the fact of its longitudinal motion, makes the handling of the bar difficult and costly.
While it is known to apply various imaging approaches to the inspection of cast or rolled Metal Flats in line, visible light imaging technologies have heretofore not been used in in-line Long Products (i.e., metal bar with a substantial length) inspection. Conventional imaging systems are not believed capable for use in inspecting metal bars and the like because the geometry of the metal bars invalidate the illumination and imaging designs that are used to enhance/capture defects on flat surfaces. FIG. 4 illustrates the differences of applying illumination and of capturing images on a flat workpiece (i.e., image line 318 converges on illumination line on flat 316) versus a round workpiece. As to the non-flat workpiece, the freedom in optical alignment and optical working ranges disappears when the object of interest does not have a flat surface. For instance, the image line 18 and the illumination line 18′ may not overlap if the light or the camera is tilted, as shown in exemplary fashion in FIG. 4. One prior art approach employs the use of area cameras to inspect the bar surfaces. However, it requires that the bar be stationary during imaging. Another prior art approach employs the use of line scan cameras, yet requiring the bar to spin for the scanning due to its flat lighting design. In order to cope with the high longitudinal traveling speed, photo-sensitive diodes, instead of imaging sensors, are used in yet another prior art. The use of photo-sensitive diodes limits the capability of detection to short, transverse defects on the bar surface. This approach is incapable of detecting long, thin defects such as seams on a steel bar.
To avoid the lighting issue, use of infrared (IR) imaging devices is reported. In this approach, IR cameras are used to capture the self-radiated light from the long products. This approach is limited to the surface defect detection solely based on surface temperature. It is known that surface voids of a hot object appear to be hotter than their neighborhoods due to the cavity theory, even though these voids are at the same temperature as their neighborhoods. This approach is further limited to its detection capability because of the focusing resolution limit of IR radiation. It is known to those skilled in the art that the optical focusing resolution is inversely proportional to the wavelength of the radiation. Typically IR cameras are nearly 10 times more expensive than a visible one and IR cameras are limited in their imaging speed due to the sensor property. As a result, this approach would not be able to accommodate the speed of today's long products.
Temperature also makes the long products different to their flat counterpart. Metal bars typically are at a higher temperature than Metal Flats. Heat dissipation of an object is proportional to the area exposed to the cooling media, such as ambient air or water spray. The area of a Metal Flat is several times larger than that of a metal bar, assuming both the flat and the bar are made of the same material and both have the same longitudinal unit density and cross section area.
It is, however, known to employ imaging-based instruments for bar gauge measurement/control (shadow measurement), bar existence/presence, and bar traveling speed measurement in the Reducing Process.
It is also known to employ electromagnetic devices, such as eddy current-based instruments, in the assessment of long products. Eddy-current based sensing systems are used for the detection of surface imperfections in the Reducing Process for in-line inspection. This approach has a high response rate, able to work in a high throughput production line environment (e.g., one kilometer of hot steel bars per minute). However, this approach has several drawbacks. First, it must be very close to the hot surface (typically less than 2.5 mm). Accordingly, it is vibration sensitive and temperature sensitive. Moreover, it is not quantitative in the sense that it is NOT able to describe the nature of the detected defect. Finally, eddy-current approaches are incapable of detecting certain types of defects. As a result, the inspection outcome from eddy current devices is not used by the metal industry for a deterministic judgment on the quality of a specific product. Rather, the output of eddy current-based instruments is only used for qualitative analysis, such as “this batch of steel bars is generally worse than the batch produced last week,” in the Reducing Process for process control purposes, for example, only.
Another approach attempted in the art employs ultrasonic sensing. This is an approach to replace the eddy current sensors with ultrasonic ones. However, many of the restrictions associated with eddy current-based instruments, such as the short working distance, apply with equal force.
Other inspection technologies used in the art include magnetic penetrant, circumflux, and infrared imaging with induction heating. The use of these technologies, however, is restricted. First, these techniques can only be used on “cold” metal bars. That is, these technologies cannot be used for in-line inspection during or shortly after hot rolling applications. Also, the metal bars must be descaled before inspection. In addition, the use of magnetic penetrant is messy and cumbersome. This process typically relies on human observation with ultra violet illumination, instead of automatic imaging and detection. The circumflux device is an eddy-current based unit, designed with a rotating detection head. Such rotating mechanism limits the application of this device in metal bar inspection with high traveling speeds, typically used at about 3 m/s. Such device is also expensive due to the moving sensing head design. The combination of induction heating and infrared imaging is based on the fact that induction current is only formed on the surface of the metal bar, and the surface defects on the metal bar will result in higher electrical resistance. Therefore, the spots with surface defects will heat up faster than other areas. There are issues associated with this approach in that (a) such faster heat up is a transient effect and thus timing (time to take images) is very critical; and (b) infrared sensors are not available for very high data rates and therefore cannot support metal bars with high traveling speed.
Of course, inspection is possible after manufacture of the metal bars. However, post-manufacturing inspection often is not possible because the product is so long and coiled up, making the bar surfaces not accessible for cold inspection technologies.
Currently, real-time inspection of metal bars manufactured with Reducing Processes is very limited. Metal bars are generally shipped from the manufacturer to the customer even if defective signals are posted by a conventional in-line eddy current inspection system. Customer complaints may therefore appear 3 to 6 months later due to surface defects on the metal bar products that are not immediately apparent to the customer. Such complaints cost the metal bar suppliers (i.e., manufacturers). The metal bar suppliers will either refund the customers for the entire coil/batch or cost share the expenses of additional labor to inspect the parts made out of the metal bar coil/batch.
There is therefore a need for an apparatus and method to minimize or eliminate one or more of the problems set forth above.